1. Field of the Invention
The invention relates to fabrication of functionalized particles useful for forming a variety of ordered, periodic structures.
2. Discussion of the Related Art
There is an increasing interest in periodic two- and three-dimensional structures, for a variety of applications, including photonics, filters, catalysts, and biocompatible materials. Of particular interest for photonics applications are periodic dielectric structures, also referred to as photonic crystals (PCs), particularly PCs exhibiting gaps in photonic band structures. Such photonic band gap (PBG) materials are discussed, for example, in P. S. J. Russell, xe2x80x9cPhotonic Band Gaps,xe2x80x9d Physics World, 37, August 1992; I. Amato xe2x80x9cDesigning Crystals That Say No to Photons,xe2x80x9d Science, Vol. 255, 1512 (1993); J. G. Fleming and S. Y. Lin, xe2x80x9cThree-dimensional photonic crystal with a stop band from 1.35 to 1.95 xcexcm,xe2x80x9d Optics Letters, Vol. 24, 49-51 (1999); B. H. Cumpston et al., xe2x80x9cTwo-photon polymerization initiators for three-dimensional optical data storage and microfabrication,xe2x80x9d Nature, Vol. 398, 51-54 (1999); and U.S. Pat. Nos. 5,600,483 and 5,172,267.
PBG materials exhibit a photonic band gap, analogous to a semiconductor""s electronic band gap, that suppress propagation of certain frequencies of light, thereby offering, for example, photon localization or inhibition of spontaneous emissions. Such structures are potentially useful as waveguides and microcavities for lasers, filters, polarizers, and planar antenna substrates. A PBG material is generally formed by combining a high refractive index dielectric material with a three-dimensional lattice of another material (or a lattice of cavities or voids) having a low refractive index, to form a three-dimensional Bragg grating. The propagation of light in the PBG structure therefore depends critically on the particular energy of the photon; photons having energy within the PBG are unable to propagate through the material, and are consequently rejected (reflected).
The photonic band structure depends on the precise details of the physical structure and on its refractive index contrast, and some difficulty has arisen in fabricating such materials. Specifically, it has been difficult to organize an extended three-dimensional periodic lattice with submicron and micron scale index contrast, particularly with high refractive index materials. (Periodicities on a submicron and micron scale, as used herein, indicate that a structure contains repeating units, the repetition occurring at a distance falling within the range 0.1 xcexcm to 100 xcexcm.)
In one approach, reflected in the above-cited references, solid materials are provided with numerous holes by mechanical techniques such as drilling, by lithographic techniques such as etching, or by selective polymerization. These approaches have provided useful results, but are limited by the ability of current processing technology to provide the necessary structure, e.g., they tend to be complicated and are only applicable to a limited number of materials and structures.
In another approach, ordered colloidal suspensions or sediments, e.g., from particles of soluble, etchable, chemically distinguishable, and relatively low refractive index materials such as polystyrene, referred to as colloidal crystals, are used as templates for infiltration or deposition of high refractive index materials in a desired structure, and the particles are then etched away or. burned out to provide the voids. See e.g., B. T. Holland et al., xe2x80x9cSynthesis of Macroporous Minerals with Highly Ordered Three-Dimensional Arrays of Spheroidal Voids,xe2x80x9d Science, Vol. 281, 538 (1998); E. G. Judith et al., xe2x80x9cPreparation of Photonic Crystals Made of Air Spheres in Titania,xe2x80x9d Science, Vol. 281, 802 (1998); and A. A. Zakhidov et al., xe2x80x9cCarbon Structures with Three-Dimensional Periodicity at Optical Wavelengths,xe2x80x9d Science, Vol. 282, 897 (1998). The infiltration/deposition has been performed, for example, by an alkoxide sol-gel technique and by chemical vapor deposition (CVD). The results attained by these methods have been interesting, but are far from providing a commercially feasible product. In particular, formation of a colloidal crystal having the desired order and periodicity over extended distances has been difficult, and thus the resulting structures often lack the requisite order for photonics applications.
Improvements in fabricating periodic three-dimensional structures are reflected in co-assigned patent applications Ser. Nos. 09/248,858, 09/248,577, and 09/312,165 (our reference, respectively, Braun 1-18-4, Braun 2-9-5, and Braun 3-6). However, techniques that provide even more improved fabrication of such structures would be desirable.
The invention provides a technique for forming functionalized particles, where such particles are readily formed into ordered, periodic structures. According to the invention, a layer of particles is formed on a substrate, a first material is deposited over at least a portion of each of the particles, and then a functionalizing agent is attached to the first material. (Functionalizing agent indicates a material capable of self-assembly to or capable of being bound to a complementary agent or material, thereby allowing control over the arrangement of functionalized particles into a desired structure. Attached indicates, for example, chemical binding, chemisorption, or even attraction.) The functionalized particles are capable of being formed into an ordered structure, by selection of appropriate complementary functionalizing agents on a substrate and/or on other particles and/or on other regions of the same particles.
In one embodiment, a portion of the first material is removed from the particle prior to functionalization to provide a selected region to which the functionalizing agent will attach, e.g., the first material is a metal deposited by electron beam evaporation, and a wet etch is performed to remove part of the deposited metal. See, e.g., FIGS 1A-1C. (Material, as used herein, indicates one or more materials deposited simultaneously or sequentially, e.g., a metal alloy or sequential deposition of two different materials.)
In another embodiment, reflected in FIGS. 3A-3D, prior to depositing the first material, a removable layer is formed over the layer of particles such that portions of the particles are exposed above the removable layer. The coverage of the first material is thereby able to be controlled by the extent to which the particles are exposed. The first material is deposited onto the exposed portions, and the removable layer is removedxe2x80x94typically before attaching the functionalizing agent.
It is possible to functionalize more than one region on the particles. For example, in an embodiment reflected in FIGS. 6A-6D, to add an additional functionalized site, the functionalizing agent of the functionalized particles is attracted onto the surface of a substrate to form a layer of the functionalized particles. Then, a removable layer is formed such that non-functionalized portions of the particles are exposed above the removable layer. A second material, which can be different from the first material, is formed on at least part of the non-functionalized exposed portions. A second functionalizing agent, which can be different from the first functionalizing agent, is attached to the second material, and the removable layer is then removedxe2x80x94typically before attaching the second functionalizing agent. Additional functionalization is also possible.
In one approach, suitable for a variety of embodiments, the particles are silica spheres, the first material consists of a layer of gold on titanium, and the functionalizing agent is a single strand DNA having a thiol end group that attaches, i.e., chemically binds in this case, onto the gold. The complementary single strand DNA is then able to be used to order the particles in a desired manner.
The functionalized particles are capable of being assembled in a variety of ways, to form a variety of structures, using principles of self-assembly.